If you're writing fiction you can't go too far wrong with the laws laid down by Mark Twain in his essay on James Fenimore Cooper's literary sins.

Here are some other things he said about writing:

You need not expect to get your book right the first time. Go to work and revamp or rewrite it. God only exhibits his thunder and lightning at intervals, and so they always command attention. These are God's adjectives. You thunder and lightning too much; the reader ceases to get under the bed, by and by.
- Letter to Orion Clemens, 3/23/1878

The time to begin writing an article is when you have finished it to your satisfaction. By that time you begin to clearly and logically perceive what it is that you really want to say.
- Mark Twain's Notebook, 1902-1903

To get the right word in the right place is a rare achievement. To condense the diffused light of a page of thought into the luminous flash of a single sentence, is worthy to rank as a prize composition just by itself...Anybody can have ideas--the difficulty is to express them without squandering a quire of paper on an idea that ought to be reduced to one glittering paragraph.
- Letter to Emeline Beach, 2/10/1868

I notice that you use plain, simple language, short words and brief sentences. That is the way to write English - it is the modern way and the best way. Stick to it; don't let fluff and flowers and verbosity creep in. When you catch an adjective, kill it. No, I don't mean utterly, but kill most of them - then the rest will be valuable. They weaken when they are close together. They give strength when they are wide apart. An adjective habit, or a wordy, diffuse, flowery habit, once fastened upon a person, is as hard to get rid of as any other vice.
- Letter to D. W. Bowser, 3/20/1880

A successful book is not made of what is in it, but what is left out of it.
- Letter to H. H. Rogers, 5/1897

No one can write perfect English and keep it up through a stretch of ten chapters. It has never been done.
- "Christian Science"